Superconductivity denotes a phenomenon in which resistance of a substance disappears when the temperature of the substance is lowered. When a substance having superconductivity is used, electric current flows in the substance without generating heat, and thus a loss of the current is not generated. This substance is referred to as a superconductor. The superconductor can pass current without resistance only in a condition of less than a transition temperature (Tc) and a critical magnetic field (Hc). Here, the maximum density of current, which the superconductor passes without resistance, is referred to as a critical current density (Jc).
A superconductive magnet, which generates a magnetic field having a high intensity, is made of the superconductor processed in a linear or tape shape. The superconductive magnet is manufactured by winding up a rod wire in various geometric coil shapes. When current flows along the rod wire, a magnetic field is generated from the coil. In case that the rod wire is made of a superconductor, a loss of the current due to resistance is not generated.
The superconductive magnet is used in spectroscopes, such as a Magnetic Resonance Imaging (MRI) and a Nuclear Magnetic Resonance (NMR). In order to exhibit regulated characteristics, the spectroscope requires a constant and stable state of the magnetic field. For the reason, superconducting joint between both ends of the superconductive magnet wound up in a coil is formed to create a closed circuit in a persistent current mode. When the proper superconducting joint is formed, current flows along the joint portion and other portions of the superconductive magnet without a loss of energy due to electrical resistance, and a magnetic field generated from the superconductive magnet is constantly maintained, thereby allowing the superconductive magnet to enter a persistent current mode in which a desired magnetic field is constantly and stabled maintained.
In U.S. Pat. No. 6,531,233 and Korean Patent Laid-open Publication No. 2001-0086623, as shown in FIG. 1, a magnetic rod wire 700 wound up in a coil and made of metal is prepared, and both ends of the magnetic rod wire 700 are mechanically joined by pressing or using a connection member 710. Further, in Korean Patent Laid-open Publication No. 1991-015512, ceramic high-temperature superconductive elements are heated to a temperature of 750˜875 degrees celsius by fuel gas or oxygen flame, and a rod, which is made of the same material as the superconductive elements and is prepared at one end of the elements, is heated, thereby allowing both ends of the elements to be joined.
In case that a superconductive rod wire coil is made of an oxide-grouped high-temperature superconductor, differently from a metallic high-temperature superconductor, since the superconductor has high brittleness, the superconducting joint cannot be achieved by butt joining, in which the superconductors contact each other and are then joined by applying pressure. When the same pressure as that, which is applied to the metallic high-temperature superconductor, is applied to the oxide-grouped high-temperature superconductor, the pressure destroys superconductive crystal grains so that supercurrent flowing in the oxide-grouped high-temperature superconductor is limited.
Further, in order to achieve superconducting joint suitable for a superconductive magnet used in the MRI and NMR, the entire portions or a joint portion of a superconductive magnet coil must be thermally treated at a high temperature. When only the joint portion of the superconductive magnet coil is thermally treated, a difference of characteristics between the thermally treated portion and other portions of the superconductive magnet coil may occur.